Adrenergic beta-agonistic drugs characteristically contain as part of their structure an ethanolamine or 2-amino-ethanol moiety. Since the chemical structures of these drugs usually comprise at least one asymmetric carbon atom, these drugs commonly exist in optically active isomeric form, with the chiral carbon atom having (R) or (S) configuration. When there is a single asymmetric carbon atom present, the beta-receptor agonists exist as individual (R) or (S) enantiomers or in racemic (RS) form, i.e. as a 50:50 mixture of (R) and (S) enantiomers.
Compounds with two chiral centers have four isomers: the RR-, SS-, RS-, and SR-isomers. Such compounds (e.g. formoterol, ractopamine) may exist in a number of forms i.e. in the pure RR or SS or RS or SR isomeric forms, or as mixtures, hereinafter called “enantiomeric pairs” of either RR/SS or RS/SR. The compound ractopamine can also exist as racemic mixtures of all four isomers (RR+SS+RS+SR) or in the form of racemic mixtures of the enantiomeric pairs (RR/SS) or (RS/SR). The isomers (RR) and (SS) are mirror images of each other and are therefore enantiomers, which have the same chemical properties and melting points. (RS) and (SR) is similarly an enantiomeric pair. The mirror images of (RR) and (SS) are not, however, super imposable on (RS) and (SR). This relationship is called diastereomerism, and (RR) is a diastereomer to (RS).
Ractopamine has the molecular formula C18H23NO3 and racemic ractopamine is typically prepared as a hydrochloride salt. Chemically, ractopamine is differs from dobutamine in the location of only one hydroxyl group, but ractopamine is not a catecholamine and is therefore not instantaneously metabolised by catechol-O-methyl transferase. Ractopamine HCl (4-hydroxy-a-[[[3-(4-hydroxyphenyl)-1-methylpropyl]amino]methyl]benzenemethanol hydrochloride) has a molecular weight of 337.85 and a molecular formula of C18H23NO3.HCl (CAS number: 90274-24-1). The racemate of ractopamine is a mixture of all four isomers in approximately equal proportions.

One form of ractopamine—the racemic mixture of all four isomers (RR/SS/RS/SR) is commercially available under the trade names PAYLEAN®, Elanco and OPTAFLEX®, Elanco and both are used as growth promotants for livestock. The RR-isomer of ractopamine is called Butopamine Hydrochloride, USAN and has extensively been tested as a cardiac stimulator for humans by Leier et al., which publications are hereby included by reference in their entirety (Thompson, M J; Huss, P; Unverferth, M D; Fasola A; Leier, C V: Hemodynamic effects of intravenous butopamine in congestive heart failure. Clin Pharmacol Ther, 1980, 28: 324-334). Butopamine was not further developed as human medication.
Although structurally identical, isomers can have different effects in biological systems: one isomer may have specific therapeutic activity while another isomer may have no therapeutic activity or may have entirely different forms of biological activity.
The pharmacological activity of beta-receptor agonists like ractopamine is to activate adrenergic beta-receptors. Activation of adrenergic beta-receptors leads to increased intracellular concentration of cyclic adenosine monophosphate (cAMP), which triggers various events in various cells and organs. Cellular responses to beta-receptor activation include for example lipolytic activity in adipose tissues, smooth muscle relaxant activity in the bronchi and increased frequency of contractions in the heart (Goodman-Gilman, The Pharmacological Basis of Therapeutics 9th Ed., 1996 McGraw-Hill ISBN0-07-026266-7.) Most adrenergic beta-receptor agonists have affinity for two or three types of adrenergic beta-receptors. Thus, both salbutamol and ractopamine have affinity for adrenergic beta-1 and beta-2 receptors, but negligible affinity for beta-3 receptors. There is no significant effect of ractopamine on adrenergic alpha-receptors (Colbert W E, Williams P D, Williams G D: Beta-adrenoceptor profile of ractopamine HCl in isolated smooth muscle and cardiac muscle tissues of rat and guinea pig, J Pharm Pharmacol 1991, 43: 844-847.) It may therefore be concluded that ractopamine does not have direct effects on adrenergic alpha-receptors in the brain.
Of the four isomers of ractopamine, which are RR-, RS-, SR- and SS-ractopamine, it is known that RR-ractopamine is the most potent, both when tested in vitro (Mills S E, Kissel J, Bidwell C A, Smith D J, Stereoselectivity of porcine β-adrenergic receptors for ractopamine stereoisomers. J. Anim. Sci. 2003, 81: 122-129) and in vivo (Ricke E A, Smith D J, Feil V J, Larsen G L, Caton J S, Effects of ractopamine HCl stereoisomers on growth, nitrogen retention and carcass composition in rats. J. Anim, Sci. 1999, 77:701-707, which publications are hereby included in their entirety by reference.) Thus, when tested for binding affinity for porcine adrenergic β-2 receptors, RR-ractopamine was about 2.5 times as active as the racemic mixture of all four isomers (Mills et al., 2003.)
Adrenergic beta-receptor agonist drugs have pharmacological and toxicological side effects that range from minor importance to major importance. Bronchial smooth muscle relaxation by adrenergic beta-2 stimulation may be a side effect of minor importance for livestock animals. However, racemic ractopamine has been found to cause CNS-mediated stress in livestock animals (Marchant-Forde J. N., et al., The effects of ractopamine on the behaviour and physiology of finishing pigs” J Anim Sci., 2003, 81: 416-422, which publication is hereby included in its entirety by reference.) This is a side effect of major importance, as racemic ractopamine is increasing the stress levels in animals during handling and transport and is causing increased mortality during transport. Stress in livestock animals, particularly in swine, is believed to induce the PSE syndrome in the animals (poor meat quality that is pale, soft and exudative, becoming dry upon cooking).
Ractopamine, having preference for adrenergic (cardiac) β1 receptors, may cause tachycardia in livestock animals by direct stimulation of cardiac β1 receptors, while R-salbutamol, having preference for adrenergic β2 receptors is not causing tachycardia in the livestock animals (Marchant-Forde J. N., et al., 2003 and London C. J., et al. 2005.) However, it is nevertheless not known, if the significant tachycardia in livestock animals by ractopamine is caused by CNS-mediated stress or by direct beta-receptor stimulation or both, but tachycardia is an unwanted side-effect, which may lead to cardiac tachyarrhythmias and increased lethality of livestock animals by sudden cardiac death (cardiac ventricular fibrillation.)
In many animals including livestock animals, birds and fish, stress manifests itself—directly or indirectly—in a range of forms extending from irritability to aggression. Stress may lead to cardiovascular side effects ranging from slightly elevated heart rate to serious tachycardia and cardiac arrhythmias, which in turn can lead to sudden death. The prevalence of stress-induced lethality varies among species; some having higher stress responsiveness than others (Odeh F. M., Cadd G. G., Satterlee D. G. Genetic characterization of stress responsiveness in Japanese quail. Poult Sci., 2003, 82: 31-35, which publication is hereby included in its entirety by reference.)
Stress in horses can be expressed in various ways, such as for example nervousness, anxiety and tachycardia and can be caused for example by heat, transportation and feed withdrawal. Stress in horses can also be induced by drugs or aggravated by drugs, such as for example adrenergic beta-receptor agonists that may be given to the horses of various reasons, such as for example as bronchodilators in heaves. CNS-mediated stress in horses may also lead to increased susceptibility for various diseases, such as for example allergic diseases or infectious diseases such as opportunistic bacterial infections. The use of an adrenergic beta-agonist that does not cause stress is particularly important in animals that are already suffering from stress or have a propensity for developing stress.
Stress in pigs is very common and some pigs have been shown to carry a specific stress-gene. Pigs that are homozygous to this gene are particularly stress-prone although heterozygous pigs are also more stress-prone than pigs that do not at all carry or express the stress-gene (Sterle J.: The Frequency of The Porcine Stress Gene in Texas Show Pigs. http://animalscience.tamu.edu, which publication is hereby included in its entirety by reference.) CNS-mediated stress in pigs can be expressed in various ways, such as for example aggression, tail-biting, and tachycardia and can be caused for example by heat, transportation, stocking density, human interventions, feed withdrawal, disease and aggression between males. Stress in pigs can also be caused or aggravated by drugs, such as for example racemic ractopamine (Marchant-Forde et al. 2003.) Porcine Stress Syndrome (PSS) is triggered when pigs are subjected to stress associated with transportation, restraint, fighting, mating, exercise or hot and humid weather. Pigs with PSS become dyspneic, hyperthermic, cyanotic, develop muscle rigidity and such animals often die. Some degree of stress can be observed in most pigs and most pigs may therefore have propensity for stress. The administration of certain drugs, such as racemic ractopamine to pigs may induce or aggravate PSS in swine. In addition to the well-known fact that stress induces increased mortality in swine, it has been demonstrated that stress has a negative effect on the quality of meat. Thus, the muscles from stress-positive pigs often show the PSE syndrome (pale, soft and exudative). This condition causes the carcasses to be classified as being of unacceptable or inferior quality, since the meat from such animals tend to become dry when cooked. (Stadler K: Porcine Stress Syndrome and Its Effects on Maternal, Feedlot and Carcass Quantitative and Qualitative Traits. The University of Tennessee, Agricultural Extension Service, PB 1606, which publication is hereby included in its entirety by reference.) The use of an adrenergic beta-agonist that does not cause stress is particularly important in animals that are already suffering from stress or have a propensity for developing stress.
Stress in ruminants can be expressed in various ways and in cattle ranging from anxiety to aggression or depression, increased body temperature and increased heart rate, and can be caused by a variety of factors, such as changes in environment, transportation, human contact, aggressive herd behaviour and changes in the herd social rankings, hunger, thirst, fatigue, injury or thermal extremes (Boissy, A. & Bouissou, M-F: Assessment of individual differences in behavioural reactions to heifers exposed to various fear-eliciting situations. Applied Animal Behaviour Science, 1995, 46:17-31; and Grandin, T.: Behavioural agitation during handling of cattle is persistent over time. Applied Animal Behaviour Science, 1993, 36:1-9, which publications are hereby included in their entirety by reference). The propensity for stress in cattle seems to affect most animals and the administration of drugs, such as racemic ractopamine may induce or worsen CNS-mediated stress in cattle and particularly in cattle that are predisposed for stress. Stress in cattle is a serious condition and may lead to decreased quality of the meat and increased lethality among the animals. The use of an adrenergic beta-agonist that does not cause stress is particularly important in animals that are already suffering from stress or have a propensity for developing stress.
As other examples of ruminants, sheep also develop symptoms of CNS-mediated stress due to the same or similar factors as described above for other species and may include but are not limited to changes in the environment, transportation, human contact, aggressive herd behaviour, hunger, thirst, fatigue, injury or thermal extremes. The symptoms of CNS-mediated (psychological) stress are similar to those of other species and include anxiety, aggression, increased body temperature or increased heart rate. The consequences of stress are similar to those described above for other species and include risk for decreased quality of meat and sudden death of the animals. The administration of drugs, such as racemic ractopamine may induce stress in sheep—particularly in predisposed animals or increase the symptoms of stress in said species. Stress in sheep can be a serious condition and may lead to decreased quality of the meat and increased lethality among the animals. The use of an adrenergic beta-agonist that does not cause stress is particularly important in animals that are already suffering from stress or have a propensity for developing stress.
As still another example, birds such as chickens ducks, geese, turkeys, ostriches, emus or quails may also develop CNS-mediated stress by doses of racemic ractopamine, corresponding to those necessary for obtaining increased muscle weight, decreased fat deposits and improved feed efficiency. Particularly, chickens in “grower houses” are suffering from stress or are predisposed to stress because of the high stocking density (up to 20,000 birds in a very confined space). Symptoms of stress in birds, such as for example chickens, ducks, geese, turkeys, ostriches, emus and quails, can be expressed in various ways, as for example, anxiety, aggression, increased body temperature, tachycardia and lethality and can be caused for example by heat, transportation, high stocking density, sudden environmental factors, feed withdrawal, injury or disease. The administration of the beta-receptor agonist racemic ractopamine may induce or increase stress in birds. CNS-mediated stress in birds—and particularly in chicken—may lead to decreased quality of the meat and increased lethality among the animals.
Stress may also manifest itself in farmed fish, such as for example barramundi, carp, cod, perch, salmon, trout and tilapia. Symptoms of stress and symptoms for predisposition (propensity) for stress in farmed fish can be observed as increased activity as for example during feeding frenzy and stress can lead to sudden death of the fish. Stress in fish can be caused for example by extreme temperatures, environmental factors, disease, parasites, handling or transportation. The administration of exogenous beta-receptor agonists may lead to stress in animals that are predisposed for developing stress or may cause a worsening of the symptoms of stress in fish, leading to decreased quality of the meat and increased lethality among the animals. The use of an adrenergic beta-agonist that does not cause stress is particularly important in animals that are already suffering from stress or have a propensity for developing stress.
Stress in animals can be monitored, judged and rated by individuals who are skilled in the art of animal psychology. In addition to monitoring and rating the behaviour of the animals, objective parameters are being used, such as for example determination of the concentration of circulating corticosteroid levels and heterophil counts. (Post J, Rebel J M J, ter Huurne A A: Physiological Effects of Elevated Plasma Corticosterone Concentrations in Broiler Chicken; An Alternate Means by which to Assess the Physiological Stress. Poultry Science, 2003, 82: 1313-1318, which publication is hereby included in its entirety by reference.) Depending on the species, stress in animals in response to exogenous adrenergic stimulation can also be monitored by parameters such as body temperature, heart rate, spontaneous motility, aggression, ease of handling and even weight loss (Marchant-Forde J. N. et al, 2003.)
The use of an adrenergic beta-agonist that does not cause stress is particularly important in animals that are already suffering from stress or have a propensity for developing stress. The use of an adrenergic beta-agonist that does not cause stress is particularly important in animals that are already suffering from stress or have a propensity for developing stress. As mentioned above, predisposition of stress in livestock animals is common and it will be advantageous to avoid the worsening of the stress in these animals that is induced by racemic ractopamine.